body{
    background-image: url("../img/学校风景4.jpg");
    background-position: center center;
    background-repeat: no-repeat;
    background-attachment: fixed;
    background-size: cover;
}
.register{
    width: 500px;
    height: 430px;
    margin: auto;
    background: lightblue;
    opacity: 0.9;
}